Period.
In fact, it's a current discussion in The Romance Room email list.
In that same vein, I found some helpful links on writing a synopsis (obviously this is more geared to fiction writing than non-fiction).
- http://www.writing-world.com/publish/synopsis.shtml
- http://www.kathycarmichael.com/synopsis.clearwater.fiction.html
- http://www.writing-life.com/fiction/synopsis.html
- http://www.fictionfactor.com/articles/synopsis.html
- http://www.charlottedillon.com/synopsis.html
I personally have used the condense each chapter/section down to one sentence, see what you have, build it from there and smooth it out method. That works well for me.